Driverless vehicles of the type involved herein are known. See U.S. Pat. No. 3,356,040 which shows a driverless vehicle and a drive wheel spring biased into frictional contact with a drive shaft. Depending upon the angular relationship between the axis of rotation of the drive wheel and the longitudinal axis of the shaft, the vehicle will move or remain stationary. It is conventional for such vehicles to have the drive wheel spring biased to a drive position.
The standard drive wheel includes a tire portion which is a smooth surfaced polyurethane having a durameter hardness of approximately 90 Shore-A. When the drive shaft is wet or oily, the conventional tire portion tends to "plane" or slide on the surface and insufficient frictional force is transmitted to drive the vehicle.
In order to solve the problem, a steel tire was tried and was partially successful because the hard surface of the steel tire would break through the liquid film on the drive shaft and provide metal to metal contact. The disadvantage of using a steel tire was excessive wear on the drive tube.
Other materials and constructions of tires were tried. Transverse and circumferential grooves were each tried on the tire portion without success. Tires were molded using 50% polyurethane and 50% of a commercially available oil absorbent material. At first, the tire did not perform satisfactorily. Thereafter, it did provide adequate drive force. The tire was removed and inspected. It was noted that the absorbent material exposed on the periphery of the tire had eroded away. Thereafter, recesses were drilled on the periphery of a conventional drive wheel having a polyurethane tire which performed very well thereby confirming that the solution to the problem was the recesses.